I have a 110v AC circuit i want to detect when it is on or off w/ a raspberry pi gpio pins.

 

A diode into 2 resistors to reduce the voltage from 170 V peak to whatever the range of your chip. 3.3v? 5v?
then a capacitor to hold the peak value for most of a second. The measured voltage will bleed off through the resistors when the input stops.

There is another method with an opto-isolator if that is necessary.

 

Or simply, connect a 110 V lamp to the 110V and watch it with a light detector shield.

 

Or plug in a $2 USB wall wart and use the 5 V output as a logic signal. Put a 47 ohm 2 W resistor across the output to shorten the detection time.

Don't the house rules mandate a galvanic isolation method?

ak